Intact microbial agents are known to have immunological effects. These effects include both immunoadjuvant efficacy and antitumor effects, in both experimentally induced and human malignancies. The active components, consisting of the peptidoglycan cell wall skeleton and trehalose dimycolate, have been isolated from mycobacteria. These active components, especially when attached to mineral oil or squalene, are known to be as active as the intact microbial agents. See, for example, E. Ribi et al., Ann. NY Acad. Science U.S.A., 277, 228-236 (1976).
The cell wall skeleton of Nocardia rubra (N-CWS) is also known to activate macrophages. Given intravenously, oil-attached N-CWS can cure some rats with experimental pulmonary metastases. See, for example, S. Sone et al., Cancer Immunology Immunotherapy, 12, 203-209 (1982). Smaller, water soluble monomeric units of the cell wall peptidoglycans have been demonstrated to be adjuvant active. Adjuvants are compounds causing stimulation of the immune system of a human or other mammal which result in an increased production of antibodies and in an enhancement of the protective reaction of the organism, e.g., against infection. Such monomeric units have also shown antitumor activity when given intravenously, for example, in mice bearing the Lewis lung carcinoma or the MCA mammary carcinoma. See, for example, G. Sava et al., Cancer Immunology Immunotherapy, 15, 84-86 (1983).
The active components of these organisms have been isolated, purified, and synthesized. These components are glycopeptides constituting a broad class of organic compounds which include a sugar part and a peptide part. Glycopeptides found in the cell are known to retain not only adjuvant activity, as evidenced by their ability to increase the antibody response, but also possess antitumor activity, as evidenced by their ability to activate macrophages to become cytotoxic and destroy tumor cells. For example, muramyl dipeptide (MDP) (e.g., N-acetylmuramyl-L-alanyl-D-isoglutamine) and a large number of MDP derivatives are known to have antitumor macrophage activation properties.
Both the in vitro and in vivo antitumor activity of mono- and disaccharide peptides is increased by their incorporation into liposomes. Lipophilic derivatives of immunomodulators and/or antitumor agents are known. Such compounds are useful for efficiently incorporating these agents into liposomes for targeting macrophages and activating macrophages to the cytotoxic state.
Both the intact microbial agents and many MDP analogues have shown an undesirable level of toxicity. Intact microbial agents, used alone or in an oil-water emulsion, such as Freund's adjuvant, can cause an increased sensitivity to histamine, granuloma formation, and hyperplasia of the liver and spleen. In particular, the administration of N-acetylmuramyl-L-alanyl-D-isoglutaminyl-L-alanyl-phosphatidylethanolamine , when given in repeated doses, has caused the undesirable toxic reaction of generalized vasculitis (See, D. G. Brown el al. in Immunotoxicology, A. Belin et al., Ed; Martin Hijhof, Pub.; 1987, pp. 219-233).
Therefore, there is a need for novel glycopeptide compounds which have improved adjuvant and/or antitumor activity, which are readily incorporated into liposomes, and which have acceptable toxicity in dosages exceeding anticipated effective human dosages.